This invention relates to communications systems and more specifically to a method and a system for transmitting connectionless datagram packets using connection-oriented networks.
The exponential rate of increase in the processing power of computers has fueled the demand for sophisticated software programs, including communications-oriented software programs, such as real-time collaborative workgroup applications. The challenge presented in finding the wherewithal to move the vast amount of data created by those applications has been met with the development and implementation of Asynchronous Transfer Mode (ATM) technology which is a connection oriented scheme. Unfortunately, implementation of ATM technology in end-points connected to ATM networks sometimes necessitates costly and significant, and sometimes wholesale, replacement of existing networking hardware and software in those end points where datagrams to be communicated are connectionless oriented. In an attempt to leverage users"" considerable existing networking investmentxe2x80x94especially in softwarexe2x80x94network designers have developed evolutionary solutions to transition today""s private lines-based local area and wide area networks to full-blown ATM connectivity with global addressing of end points. Those solutions include the so-called xe2x80x9cClassical Internet Protocol (IP) over ATMxe2x80x9d method, the xe2x80x9cLocal Area Network Emulationxe2x80x9d proposal and the xe2x80x9cRouting Over a Large Cloudxe2x80x9d method that are summarized by Chao et. al in the article xe2x80x9cIP on ATM Local Area Networksxe2x80x9d, IEEE Communications Magazine, August 1994, pp. 52-59, hereby incorporated by reference as if fully set forth herein.
Referring to FIG. 1 prior art Classical IP over ATM involves three general steps. First, assuming no ATM connection between the source IP host 110 and a destination host, source host 110 sends an ATM Address Resolution Protocol (xe2x80x9cARPxe2x80x9d) request to IP-ATM-ARP server 120 on a pre-established connection with a well known Virtual Channel Identifier (xe2x80x9cVCIxe2x80x9d). Server 120 resolves the destination IP address to an ATM address and returns the ATM address to source host 110. Second, Q.2931-based signaling software 112 in source host 110 generates a SETUP message with the destination ATM address as a parameter, and an ATM connection is established through a series of ATM switches 130. The virtual path/virtual channel connection identifier (xe2x80x9cVPI/VCIxe2x80x9d) to be used on the ATM interface from the sending node, is returned in a CONNECT message if it was not specified in the SETUP message. Finally, the IP address to VPI/VCI mapping is stored in cache 111 in source host 110. IP packets with the given destination ATM address are now routed on the VPI/VCI specified in the mapping table. As a result of this scheme, the ATM interface equipment serves as a subnet and a physical channel for communicating IP datagrams.
Upon receiving a Q.2931 signaling SETUP message, the destination host (or gateway/router) sets the specified VPI/VCI to deliver calls to the ATM Adaptation layer (xe2x80x9cAALxe2x80x9d). Connections are released when the cache manager in a sending node times out on an IP-address to VPI/VCI mapping entry and decides to remove the entry by issuing a RELEASE message.
LAN emulation (LE) on ATM today works much like the xe2x80x9cClassical IPxe2x80x9d case except the ATM ARP server is replaced with an LE-ARP server. In general, the network protocol layer sends a network Protocol Data Unit (PDU) and next-hop address to The Medium Access Control (xe2x80x9cMACxe2x80x9d) encapsulation layer for the case of the Ethernet, where traditional ARP is performed to obtain the MAC address. The network PDU is encapsulated into a MAC frame and then passed to the ATM layer. The source host extracts the MAC address from the frame and uses the LE-ARP server to map it to an ATM address. Signaling software on the host then sets up a connection using the ATM address. The VCI obtained from this connection is then cached in an ATM address-to-VCI cache for future use. The frame is then segmented into one or more ATM cells and sent on this VCI. After a period of inactivity on the VCI, the connection times out and is released.
The third prior art scheme is Routing Over a Large Cloud (xe2x80x9cROLCxe2x80x9d). A collection of end-points, routers or hosts, connected over a fabric such that direct communication can be established between any pair of end-points subject to policy restrictions employed within the fabric is generally referred to as a xe2x80x9cLarge Cloud.xe2x80x9d An ATM fabric might typically consist of a set of private and public ATM networks, interconnected via User Network Interfaces (UNI) or Private Network to Network Interfaces (PNNI). While the Classical IP model, when IP networks are overlaid over an ATM-based large cloud, requires end-points that do not share the same network prefix to communicate via an intermediate router, ROLC enables end-point communication directly at the ATM level (e.g. via an ATM Switched Virtual Connection (xe2x80x9cSVCxe2x80x9d)) even when end-points do not share the same network prefix. There are several techniques for routing over large clouds. One technique is based on the Next Hop Resolution Protocol (xe2x80x9cNHRPxe2x80x9d). NHRP is used to return a binding between a destination connectionless address and the corresponding ATM address to a requesting source connectionless host. Current work in the Internet Engineering Task Force (IETF) is considering two modes of NHRP server operation: server mode, where one server may cover several subnets; and fabric mode, where there is an NHRP server associated with each router. In the first case, the servers have their own tables for forwarding NHRP requests and would be used in early deployment of NHRP while in the latter case, the same tables that are used to find the next hop address at the routers are used to forward the NHRP requests.
All of these techniques, Classical IP, LE-ARP and ROLC propose a two-step method of a) mapping a connectionless destination address such as an IP address to a connection-oriented destination address, such as an ATM address, and b) setting up and establishing a connection over a network to permit exchange of data between a source node and a destination node. As used herein a node includes an end host, router or bridge. In all three solutions, IP addresses are mapped to ATM addresses using a form of ARP. Connections are most commonly established with signaling protocol such as ITU-T (Q. 2931), found in ITU-T, xe2x80x9cDraft Text of Q. 2931,xe2x80x9d November-December 1993, hereby incorporated by reference as if fully set forth herein, and the ATM Forum, found in the ATM Forum, ATM User-Network Interface Specification, Version 3.1 (Jul. 21, 1994).
Of particular significance is the fact that these solutions require the source and destination hosts to be loaded with signaling software to obtain datagram-to-ATM address resolution services from the network, and perform signaling message generation and processing functions for establishing a connection, including tracking all the VCIs assigned to all associated connections. These solutions expend a significant amount of network resources devoted to the processing of the considerable number of signaling messages generated for address resolution and connection request functions and delay the call setup procedure associated with these procedures.
Accordingly the present invention is directed to a communications system that combines the address resolution function and the connection setup function in a third-party connection control and which operates in conjunction with channel management at connection-oriented routing points to enable quick connection setup procedures and reduce or eliminate the need for endpoint signaling software for communicating connectionless datagrams over connection-oriented networks.
In one embodiment of the invention, a source host that has a datagram to transmit to a destination host checks its internal storage devices to determine whether a connection is already established between it and that destination host. In the absence of such a connection, the source host sends a request containing the destination host connectionless address to a connection request server located at a well known VCI. The connection request server is arranged to perform the address resolution functions as well as the connection setup functions in a single round trip communication between transmitting host and server, in contrast to the two round trips required by the prior art. Specifically, the server maps the datagram or connectionless packet address into an ATM address and then uses this address to establish a connection to the destination host. Thereafter, the connection request server returns a VCI to the transmitting host that identifies the connection for exchange of subsequent data packets.